Typechecker / OverloadedStrings question 7.8 vs. 7.10

[Michael Karg]

Hi devs,

in the followin snippet:

{-# LANGUAGE OverloadedStrings #-}
import  Data.Char (isAlphaNum)
import  Data.ByteString.Char8 as BS (all)
main =
    print $ check str
  where
    check = BS.all (\x -> x `elem` valid || isAlphaNum x)  -- Line 7
    valid   = "$_-"                     -- :: String
-- Line 8
    str      = "foo_bar123"

GHC 7.10 fails with the following errors (whereas 7.8 compiles without
complaining):

ghc --make "Testcase.hs"
[1 of 1] Compiling Main             ( Testcase.hs, Testcase.o )
Testcase.hs:7:31:
    No instance for (Foldable t0) arising from a use of ‘elem’
    The type variable ‘t0’ is ambiguous
   (...)

Testcase.hs:8:15:
    No instance for (Data.String.IsString (t0 Char))
      arising from the literal ‘"$_-"’
    The type variable ‘t0’ is ambiguous
    (...)

Uncommenting the -- :: String type annotation (line 8) makes the snippet
acceptable to the typechecker however.

So Foldable [] and  [Char] should be possible to infer, given the evidence
of 'isAlphaNum x', as obviously happens with GHC 7.8. My question is, how
or why does the 7.10 typechecker behave differently? Is this intentional,
or does this qualify for a trac ticket?

Thanks for looking into this,
Michael



PS: The ByteString part is just there since the snippet is taken out of one
of my projects. The following (modified) code only typechecks on 7.10 with
both type annotations uncommented:

{-# LANGUAGE OverloadedStrings #-}
import  Data.Char (isAlphaNum)
main =
    print $ check str
  where
    check   = all (\x -> x `elem` valid || isAlphaNum x)
    valid     = "$_-"               -- :: String
    str        = "foo_bar123"    -- :: String

The errors here are (1) no instances for Foldable and (2) no instances for
IsString.
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